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The Digestive System

6.1.1 Why digest food? Food consists of:

– Carbohydrates

– Lipids

– Proteins

– Nucleic acids

– Minerals

– Vitamins

– Water

These are large, insoluble polymer

molecules. They must be digested into

monomers small enough to pass

through the cell membrane of the cells

lining the digestive tract so they can be

absorbed in to the blood stream

Can be absorbed directly (because of

small size), so do not need to be

“digested”

6.1.2 Enzymes and digestion

• Digestion involves hydrolysis of food

molecules

breaking

apart

molecules by

adding water

Hydrolysis of lactose (a

disaccharide) into glucose and

galactose (both monosaccharides)

with the addition of water

• Digestive enzymes catalyze the hydrolysis of

insoluble food molecules to soluble end

products

Increase the rate

The enzyme sucrase

catalyzes the hydrolysis of

sucrose (a disaccharide)

into glucose and fructose

(both monosaccharides)

with the addition of water

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Digestive Enzymes

Break down complex substances into simpler

substances that can be absorbed by the body

Digestive Enzymes

Complex proteins

Complex sugars

Fat molecules

Amino acids

Glucose

Fatty acids

6.1.2 Enzymes and digestion

Digestive enzymes work best at 37⁰ C (body temp)

• Reactions occur faster at

higher temperatures, but the rate of denaturation of enzymes also increases at higher temperatures.

• High temperatures break the bonds important for the tertiary structure of the enzyme.

• This destroys the active sites and therefore makes the enzyme non-functional.

Rapid

denaturation

Temperature (°C)

En

zym

e a

ctivity

Too cold for

the enzyme

to operate

Optimum temperature

for enzyme

6.1.3 Examples of digestive enzymes

Class of

Enzyme

Example Source Substrate Product Optimal

pH

Amylase digest

carbohydrates

Salivary

amylase

Salivary

glands

Starch Maltose

(disaccharide)

7-8

Protease digest proteins

Pepsin Stomach

cells

polypeptides Shorter

polypeptides

2-3

Lipase digest fats

Gastric

lipase

Stomach

cells

triglyceride Glycerol and

3 fatty acids

2-3

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Parts of the Alimentary Canal The Digestive System

Consists of the alimentary canal (the long tube)

and several accessory organs.

Parts of the

Alimentary Canal

mouth

pharynx

esophagus

stomach

anal canal

large intestine

small intestine

6.1.4 Digestive System

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Organs of the Digestive

System

Organs of the

Digestive System Parotid

salivary

gland

Pharynx

Esophagus

Stomach

Pancreas

Small

intestine

Rectum Anal canal

Large intestine

Duodenum

Gall bladder

Liver

Submandibular

salivary gland

Sublingual

salivary gland

Tooth

Tongue

Mouth

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Tracing Toast mouth pharynx epiglottis esophagus

stomach

duodenum

jejunum

ileum

cecum ascending

colon

transverse

colon

descending

colon

sigmoid

colon

rectum

anus feces

toast

Tracing a piece of

toast through the

alimentary canal

Animation of digestive system

6.1.5 Function of the Stomach

1. Mechanical Digestion

• Muscle contractions

break apart and mix

food

• bolus chyme

2. Chemical Digestion

• Enzymes and HCl digest

chyme

Mass of

solid food

Semi-fluid

mass of

partly

digested

food

6.1.5 Function of the Small Intestine

1. Digestion by

enzymes in the

duodenum (first 50

cm)

2. Absorption of

nutrients into the

blood stream (see

6.1.7)

6.1.5 Function of the Large Intestine

Removes water and minerals

6.1.6 Absorption and Assimilation

ABSORPTION

Transport of molecules

from the intestine to

the blood stream

ASSIMILATION

Incorporation of

molecules from the

blood stream into

cellular structures

6.1.7 Structure of Villus

The small intestine has large folds which

increase the surface area for

absorption. One finger-like projection is

called a villus (plural: villi).

6.1.7 Structure of Villus

A single layer of epithelial cells line the villi.

These cells have microvilli that increase

the surface area for absorption

6.1.7 Structure of Villus

Each villus has a capillary bed that absorbs

small molecules from the small intestine

and transport the molecules in the blood

stream

6.1.7 Structure of Villus

The lacteal absorbs fats that pass through

the villi epithilium.

H.2.1 Digestive Juices and Glands

• EXOCRINE glands are different than ENDOCRINE glands

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Secrete other

stuff (sweat, oil,

wax, enzymes

etc) into ducts

(a pipe or tube)

Secrete

hormones

directly into the

blood stream

Digestive “Juices” are…

• ENZYMES

• Good time to review

• Catalyst

• Lower activation energy

• In the case of digestion, this is catabolic

rxns

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• Salivary glands in the mouth

– Secrete amylase

– Breaks starch (polysaccharide) into maltose

(disaccharide)

• Gastric glands in the stomach wall

– Secrete pepsin

– Break proteins into smaller polypeptide chains

• Pancreas

• Glands in the wall of the small intestine

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H.2.2 Structure of Glands

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• Exocrine glands contain a glandular portion and a duct

portion.

• Secretory cells form a single layer around the duct.

• The secretory cells release the secretion into the

lumen of the duct.

• Ducts open onto surfaces such as the skin or another

cavity (mouth, alimentary canal).

• A good additional resource can be found here

H.2.3 Composition of Digestive Juices

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GLAND COMPOSTION of “JUICE”

Salivary gland

(mouth) Water, salts, amylase, mucus

Gastric glands

(stomach) Water, mucus, HCl, enzymes (i.e.

pepsin)

Pancreas Water, bicarbonate (buffers acid from

stomach), enzymes (i.e. amylase,

lipase)

Small intestine Enzymes (i.e. carbohydrases to break

down carbohydrates)

H.2.4 Control of Gland Secretions The activities of the digestive system are regulated by

both hormones and neural reflexes.

HORMONES

• The physical presence of food in

the lower region of the stomach

stimulates the endocrine cells

within then stomach wall to

release the protein hormone

“gastrin”.

• Gastrin is released into the blood

and travels back to the heart and

through the arteries only to return

to the digestive system to induce

the secretion of gastric juice.

NEURAL

• Nerves come to the digestive

organs from the brain or the spinal

cord.

• The nervous system is triggered

by the senses (touch, smell, sight,

feeling…)

• The nerves release chemicals

which cause the speed up or

delay of the movement of food

and the production of juices by the

digestive organs.

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H.2.5 Membrane Bound Enzymes

• Many digestive enzymes are embedded in the

membrane of cells that line the digestive tract.

Inside the cell

Outside the cell

(active site of

enzyme oriented

towards the middle of

the alimentary canal).

Cell membrane

Being fixed to the

membrane is efficient

since the enzyme is not

removed (can be

reused) and can be

linked to secondary

functions including

membrane transport.

H.2.6 Cellulose Digestion

• Cellulose is the major

constituent of the plant cell

wall.

• Humans cannot digest

cellulose.

– We don’t produce the

cellulase enzymes

– We don’t carry the bacteria

in the gut which produce

cellulase (like cows do)

• Cellulose remains

undigested and is part

of feces

H.2.7 Pepsin and Trypsin

• Pepsin and trypsin are both protease enzymes (they

break proteins into smaller amino acid strands)

– Pepsin is produced by stomach exocrine cells

– Trypsin is produced by pancreas exocrine cells

H.2.7 Pepsin and Trypsin

• Both are initially synthesized as inactive precursors.

• Both are chemically altered after secretion to

produce the active forms of the enzyme.

• This prevents self digestion of the cells that produce

pepsin and trypsin.

Triggered by acid conditions

of the stomach

Enzyme bound to the small

intestine membrane

H.2.8 Stomach Ulcers and Cancer

• The stomach has an acidic

environment caused by the

secretion of HCl.

– The acid is a barrier to

infection from

microorganisms ingested with

food.

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H.2.8 Stomach Ulcers and Cancer

• To protect the stomach wall from it’s own acid and

digesting enzymes, a mucus lining covers the

surface.

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H.2.8 Stomach Ulcers and Cancer

• Stomach ulcers are areas where the mucus layer

has eroded, leaving the stomach muscle layers

unprotected and exposed to gastric acids and

digestive enzymes.

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H.2.8 Stomach Ulcers and Cancer Stomach cancer can develop from ulcers (however, having an

ulcer does not mean you will develop cancer.)

H.2.8 Stomach Ulcers and Cancer

• Stomach ulcers occur with an infection of the

bacterium Helicobacter pylori.

• YES! Ulcers are caused by an infectious disease.

• Check out this site for additional information…

– No. Really. Read this information.

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H.2.9 Lipid digestion in a

hydrophilic medium

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